In a typical radio communications system, radio communications terminals, referred to as radio terminals or user equipment terminals UEs, communicate via an access network with other networks like the Internet. For example, a radio access network (RAN) in a cellular communications system covers a geographical area which is divided into coverage cells, with each cell being served by a base station, e.g., a radio base station (RBS), which in some networks is also called a “Node B” or an evolved Node B “eNodeB.” Each base station typically serves several cells. One common deployment is 3-cell base station installations, where a base station serves three cells. Other wireless systems, like WiFi systems, employ access points (APs) to provide network access to wireless terminals. For simplicity, wireless access points, radio base stations, and the like are referred to generally as base stations and user equipment terminals, access terminals, and the like are referred to generally as radio terminals.
A base station communicates over the air interface operating on radio frequencies with the radio terminals within range of the base station. The radio signals may either be dedicated signals to and from specific radio terminals, multicast signals intended for a subset of the radio terminals in a cell or coverage area, or broadcast signals from the base station to all radio terminals in a cell or coverage area. For simplicity, a cell is understood to include a radio coverage area or the like. A base station broadcasts information to all the radio terminals in a cell using the broadcast channel of the serving cell. Each cell is identified by a cell identifier within the local radio area, which is broadcast in the cell.
As the mobile station moves around in the region covered by the network, at each time moment, it is connected via a radio link to the cell that can provide the connection with “least cost,” where the cost is often related to the signal power needed to transmit over the radio link. Often, but not necessarily, the serving cell is the cell physically closest to the mobile station. The mobile station makes measurements based on signals it receives from the serving and neighboring base stations. The mobile station reports the measurements to its serving base station, and based on those measurements, the network may perform a handover (HO) procedure to transfer the mobile station's connection from one cell to another. In some systems, the mobile station can maintain a connection simultaneously over several cells, using several radio links in parallel.
The work of specifying the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) consisting of the Long Term Evolution (LTE) and System Architecture Evolution (SAE) concepts is currently ongoing within the 3rd Generation Partnership Project (3GPP). To provide seamless mobility, e.g., in areas where E-UTRAN does not provide good coverage/quality, it is necessary to be able to handover (HO) mobile stations (MSs) in E-UTRAN to an alternative Radio Access Technology (RAT) such as GSM EDGE Radio Access Network (GERAN) or Universal Terrestrial Radio Access Network (UTRAN) with better coverage. It is also desired for a mobile station (MS) or UE served by 2G (e.g. GERAN) or 3G (e.g. UTRAN), to switch to E-UTRAN once the MS/UE is within the coverage of E-UTRAN. The latter is desired since higher data rates are offered by E-UTRAN enabling services with greater bandwidth requirements. Handover between two different RATs is referred to as an inter-RAT (IRAT) handover (HO).
An important focus area in E-UTRAN standardization work is to ensure that the new network is simple to deploy and cost efficient to operate. The vision is to move the new system towards self-optimizing and self-configuring in many aspects. One aspect that would benefit from self-optimization and self-configuration is the triggering of the measurements of other RATs that are needed for IRAT mobility, e.g., when handing over a mobile station from E-UTRAN to UTRAN. If the IRAT measurement triggering is too early, this could lead to early IRAT HOs that may result in lower data rates for the users and inefficiency. If the IRAT measurement triggering is too late, this leads to a too short measurement time for the mobile stations before the signal strength or quality of the serving cell is too low and may result in Radio Link Failures (RLFs). It would be desirable to provide automatic adjustment of the IRAT measurement triggering thresholds.